Antipsychotics are widely used for treating psychiatric disorders including schizophrenia, mood, bipolar disorders and autism. Most second generation antipsychotic medications have been associated with substantial weight gain and metabolic disturbances, increasing the risk for premature death (Lencz and Malhotra, 2009; Muller and Kennedy, 2006). The high prevalence of over 30% of treated individuals experiencing significant weight gain makes these side effects one of the leading causes of patient non-compliance leading to increased treatment costs. There are currently no biomarkers available for antipsychotic-induced weight gain (AIWG) and the strongest predictor remains a positive family history of AIWG in first-degree relatives. Twin and family studies have consistently pointed to high heritability suggesting a possible role of genetic factors in AIWG (Gebhardt et al., 2010).
GLP-1
Glucagon-like peptide 1 (GLP-1) is an important peptide involved in central regulation of food intake as well as in peripheral glucose regulation. GLP-1 is an incretin hormone which is released from small intestinal L-cells together with glucose-dependent insulinotropic polypeptide (GIP) following food intake. It acts on G-protein coupled GLP-1 receptors (GLP-1R), which are widely expressed in the central nervous system (CNS), pancreas, heart, gastrointestinal tract, kidneys and other tissues (Phillips and Prins 2011). GLP-1 reduces food intake, likely mediated through inhibition of central AMP-activated kinase following activation of GLP-1R in the hindbrain (Burmeister et al., 2013).
Besides augmenting insulin secretion, GLP-1 inhibits glucagon secretion and delays gastric emptying (Phillips and Prins 2011). The insulinotropic effect depends on blood glucose levels (Fu et al. 2013), and impaired GLP-1 induced insulin secretion has been found in type 2 diabetes patients (Herzberg-Schafer et al. 2012). Interestingly, an antipsychotic-like effect of a GLP-1 receptor agonist has recently been described in a mouse model (Dixit et al. 2013).
Several previous studies have investigated effects of antipsychotics on GLP-1. Antipsychotics with high or medium-high risk for antipsychotic-induced weight gain (AIWG) such as olanzapine (Smith et al. 2011), clozapine or quetiapine (Smith et al. 2009) have been shown to decrease GLP-1 levels in rat models. These effects seem to occur after a longer treatment period, since studies with short-term treatment did not show an impact of olanzapine on GLP-1 levels (Vidarsdottir et al. 2010; van der Zwaal et al. 2012). Recent research has indicated a beneficial effect of GLP-1 analogs for treatment of AIWG in animal models (Lykkegaard et al. 2008). Clinical studies showed that GLP-1 analogs were effective to induce weight loss not only in subjects with type 2 diabetes (Flint et al. 2013), but also in non-diabetic patients (Astrup et al. 2012; Vilsboll et al. 2012). The gene encoding GLP-1, GCG, is located on chromosome 2q36-37. GCG encodes a preproprotein which is cleaved into four different proteins involved in glucose homeostasis (glucagon, GLP-1, GLP-2, oxyntomodulin). Genetic variation in GCG has previously been associated with weight, insulin, GLP-1 and glucagon levels (Torekov et al. 2011). The human GLP-1 receptor gene GLP1R is located on chromosome 6p21. Variation in this gene has been associated with morning cortisol levels (Sheikh et al. 2010) and altered insulin secretion following GLP-1 infusion (Sathananthan et al. 2010). In animal models, genetic variation in GLP1R influenced food intake (Kumar et al. 2007) and gastric emptying (Kumar et al. 2008). On the other hand, glplr-deficient mice showed normal feeding behavior in an earlier study (Scrocchi et al. 1996). Despite these preliminary findings, GCG and GLP1R are interesting candidate genes for AIWG due to their implication in food intake and glucose metabolism.
Orexin/Hypocretin
The orexin system includes the orexin gene coding for pre-pro-orexin which is cleaved into two polypeptides, Orexin A (OXA, hypocretin 1, 33 amino acids) and Orexin B (OXB, hypocretin 2, 28 amino acids). The biological action of the orexin peptides is mediated through two G-protein coupled receptors orexin receptor 1 (OX1R or HCRTR1) and orexin receptor 2 (OX2R or HCRTR2; (Sakurai and Mieda 2011; Kukkonen 2013; Perez-Leighton et al. 2013)). OXA has higher affinity for OX1R whereas OXA and OXB have equal affinity for OX2R. Orexins are primarily expressed in the lateral hypothalamic area, a region associated with feeding behavior and arousal. Decreased extracellular glucose levels activate orexin neurons whereas increased glucose concentrations have the opposite effect (Yamanaka et al. 2003; Burdakov et al. 2005). Similarly, the orexigenic peptide ghrelin activates 60% of orexin neurons, whereas the anorexigenic peptide leptin inhibits most orexin neurons (Yamanaka et al. 2003). Increased level of orexin mRNA is observed in fasting conditions and intracerebrovascular (ICV) injection of orexin during light period induces feeding behavior in rats and mice. Furthermore, ICV injection of anti-orexin antibody reduces food intake (Yamada et al. 2000). In accordance with this observation, mice lacking orexin neurons exhibit hypophagia, lower levels of spontaneous physical activity (SPA) and develop late-onset obesity on a regular diet (Hara et al. 2001; Akiyama et al. 2004). Furthermore, overexpression of the pre-pro orexin gene leads to resistance to obesity induced by consumption of high fat diet. This protective effect has been primarily attributed to increased energy expenditure (Funato et al. 2009; Perez-Leighton et al. 2013).
Orexin receptors are expressed in several regions in the brain. OX1R, compared to OX2R, are predominant in the locus coeruleous, paraventricular thalamic nucleus and bed nucleus of the stria terminalis. OX2R are mainly expressed in the arcuate nucleus (ARC), paraventricular nucleus and lateral hypothalamic area (Marcus et al. 2001; Funato et al. 2009). The OX2R has been shown to play a major role in preventing high fat diet induced obesity and insulin insensitivity in mice (Funato et al. 2009). Similarly, OX2R agonist administration to wildtype-mice on a high fat diet suppressed food intake and led to significantly less fat mass. In the same study mice with OX1R deletion showed improved glucose tolerance and insulin sensitivity on a high fat diet suggesting that OX1R may also have a role in mediating the effect of high fat diet on glucose metabolism (Funato et al. 2009). Overall OX2R appears to have a major role in adverse dietary conditions with OX1R making minor contribution. The orexin gene and its receptors have also been associated to narcolepsy in mice, dogs and humans (Kukkonen 2013). Interestingly, individuals with narcolepsy have decreased caloric intake but have a higher body mass index and increased incidence of metabolic syndrome (Schuld et al. 2000; Nishino 2007).
The orexin system is modulated by leptin via its receptors, especially OX2R (Funato et al. 2009), and sends excitatory signals to neuropeptide Y (NPY) expressing neurons in the ARC (Muroya et al. 2004). In addition, it has been shown that the orexin system also interacts with endocannabinoids as injection of the cannabinoid receptor type 1 (CB1) antagonist, rimonabant, abolishes feeding induced by intracerebroventricular orexin-A injection (Crespo et al. 2008). Recently Cristino et al., (2013), reported that in murine models of obesity (leptin deficient), increased endocannabinoid synthesis causes activation of CB1 receptors (Cristino et al. 2013). This reduces inhibition of orexinergic neurons and enhances orexin-A release leading to hyperphagia and increased body weight gain. Thus, the orexin system interacts with both NPY and the CB1 expressing neurons.
Antipsychotics associated with weight gain increase activity in orexin neurons compared to antipsychotics with no weight gain liability (Fadel et al. 2002). In addition, antipsychotics associated with higher risk of weight gain (e.g. clozapine and olanzapine) activated orexin neurons significantly more than antipsychotics with relatively less AIWG risk (e.g. risperidone). Similarly, in female Sprague Dawley rats injected with olanzapine 50% of the neurons activated in the perifornical region of lateral hypothalamus are orexin A positive (Stefanidis et al. 2009). This suggests that antipsychotics with weight gain liability modulate orexin neurons. However, the impact of genetic variation in the orexin system on AIWG has not been investigated to date.
NDUF S1
The NDUFS1 gene (NADH dehydrogenase (ubiquinone) Fe—S protein 1, 75 KDa) is part of the complex I of OXPHOS. This gene encodes the largest and one of the “core subunits” of this complex and the protein is located in the iron-sulfur fragment of the enzyme complex (Smeitink et al., 1998). NDUFS1 is part of the hydrophilic arm of the complex which is responsible for the transfer of electrons (Finel, 1998, Scola et al., 2013). Reduced levels of NDUFS1 mRNA and down-regulation of the protein in postmortem brain from schizophrenia patients have been reported (Maurer et al., 2001; Prabakaran et al., 2004).
Mutations in NDUFS1 have been associated with isolated complex I deficiency (Hoefs et al., 2010), and dysfunction in the cellular oxidative metabolism with increased mitochondrial Reactive Oxygen Species (mROS) production (luso et al., 2006). The effect of variants on mROS production may be of special importance since it may influence the energy homeostasis in the hypothalamus. For example, mROS are involved in the regulation of the ATP-dependent potassium channel in POMC neurons, an important step to neuronal depolarization and downstream events that will lead to decreased food intake. Besides that, in NPY neurons, the buffering of mROS appears to be crucial to keep active the ghrelin-dependent gene expression and downstream events to stimulate food intake.
TSPO
The translocator protein-18 kDa (TSPO, chr22:43547520-43559248 Genome Reference Consortium Build 37) is a housekeeping gene. While the precise functions of TSPO are an active area of research, it is known to play a key role in steroid biosynthesis. TSPO is expressed by many tissues throughout the body, and at particularly high levels in steroidogenic tissues such as the adrenal glands and gonads. In the brain, TSPO is expressed selectively by activated microglia and reactive astrocytes, mediators of the brain's inflammatory response, which has led to the use of TSPO as an in vivo marker of neuroinflammation in PET imaging studies (reviewed by Venneti et al., 2013). At the subcellular level TSPO is localized primarily to the outer mitochondrial membrane, where it forms a multimeric complex with voltage-dependent anion channel (VDAC) and adenine nucleotide transporter (ANT) (McEnery et al., 1992).
There is evidence that TSPO plays a role in weight regulation, possibly through its effect on mitochondrial metabolism. In the leptin-deficient ob/ob mouse, an established animal model of obesity, increased TSPO binding capacity was observed in the hippocampus and hypothalamus (Giannaccini et al., 2011). Furthermore, TSPO ligands PK1195 and Ro5-4864 were recently identified as key regulators of whole-body energy control in zebra fish and mice (Gut et al., 2013). In high-fat diet induced obese mice, PK1195 treatment significantly lowered lipid accumulation in the liver, free and LDL cholesterol, and blood glucose levels (Gut et al., 2013). Taken together, these data suggest that TSPO may be a key factor in regulating energy homeostasis and body weight.
The atypical antipsychotic clozapine has been shown to increase TSPO binding capacity in the hippocampus and hypothalamus as well as in steroidogenic tissues in rats, and this increased binding capacity corresponded with increased steroid synthesis in vitro (Danovich et al., 2008). Interestingly, neurosteroids mediate response to clozapine and olanzapine in animal studies (Marx et al., 2000, Ugale et al., 2004, Marx et al., 2003, Marx et al., 2006), and human studies suggest that adjunct treatment with the neurosteroid pregnenolone improves symptoms of schizophrenia (Marx et al., 2009, Marx et al., 2011).
There is a need in the art to identify genetic markers associated with weight gain. Further, there is a need in the art for genetic markers associated with antipsychotic-induced weight gain. Further, there is a need in the art for genetic diagnostic markers for antipsychotic-induced weight gain that provide physicians and other health care professionals with the opportunity to provide educated decisions for prescribing medications in treatment regimens. Moreover, there is a need in the art for personalized medicine approaches that lower the risk of developing antipsychotic induced weight gain and related ailments such as diabetes and cardiovascular disease.